xprofile slugs for search

The specialisms list is a clickable list of all the specialisms (an x-profile field) on the site, for example the link for “police” looks like this…<li><a href='members/?s=Police'>Police</a></li>
This link, when clicked will search the site members and return a list of all members who have the word police in their profiles.
I would like to specify that only members whose “Specialism” is listed as police to be returned.
I’m assuming this will be acheivable through editing the search link to something like members/?s="specialism=police" or similar but I’ve tried as many combinations of this as I can and can’t figure it out.
Any tips massively appreciated
thanks